This application is a continuation-in-part of application Ser. No. 08/775,827 filed Dec. 31, 1996, now abandoned, for xe2x80x9cMethod and Apparatus for Reducing Defibrillation Energy.xe2x80x9d
This invention relates generally to a defibrillation method and apparatus, and more particularly to a method and apparatus for reducing the electrical energy delivered by an external defibrillator. xe2x80x9cDefibrillatorsxe2x80x9d include manual defibrillators, semi-automatic defibrillators and automatic defibrillators. This invention also relates to a method and apparatus for dynamically changing the operation of a defibrillator when treating a pediatric patient.
Sudden cardiac death is the leading cause of death in the United States. Most sudden cardiac death is caused by ventricular fibrillation (xe2x80x9cVFxe2x80x9d), in which the heart""s muscle fibers contract without coordination, thereby interrupting normal blood flow to the body. The only known effective treatment for VF is electrical defibrillation, in which an electrical pulse is applied to the patient""s heart. The electrical pulse must be delivered within a short time after onset of VF in order for the patient to have any reasonable chance of survival. Electrical fibrillation may also be used to treat shockable ventricular tachycardia (xe2x80x9cVTxe2x80x9d). Accordingly, defibrillation is the appropriate therapy for any shockable rhythm, i.e., VF or shockable VT.
One way of providing electrical defibrillation uses implantable defibrillators, which are surgically implanted in those patients having a high likelihood of experiencing VF. Implanted defibrillators typically monitor the patient""s heart activity and automatically supply the requisite electrical defibrillation pulses to terminate VF. Implantable defibrillators are expensive, and are used in only a small fraction of the total population at risk for sudden cardiac death.
External defibrillators send electrical pulses to the patient""s heart through electrodes applied to the patient""s torso. External defibrillators are typically located and used in hospital emergency rooms, operating rooms, and emergency medical vehicles. Of the wide variety of external defibrillators currently available, automatic and semi-automatic external defibrillators (referred to collectively as xe2x80x9cAEDsxe2x80x9d) are becoming increasingly popular because they can be used by relatively inexperienced personnel. Such AEDs are also especially lightweight, compact, and portable. AEDs are described in U.S. Pat. No. 5,607,454 to Cameron et al. entitled xe2x80x9cElectrotherapy Method and Apparatusxe2x80x9d and PCT Publication No. WO 94/27674 entitled xe2x80x9cDefibrillator with Self-Test Featuresxe2x80x9d, the specifications of which are incorporated herein.
AEDs provide a number of advantages, including the availability of external defibrillation at locations where external defibrillation is not regularly expected, and is likely to be performed quite infrequently, such as in residences, public buildings, businesses, personal vehicles, public transportation vehicles, etc. Although operators of AEDs can expect to use an AED only very occasionally, they must nevertheless perform quickly and accurately when called upon. For this reason, AEDs automate many of the steps associated with operating external defibrillation equipment, and the operation of AEDs is intended to be simple and intuitive: AEDs are designed to minimize the number of operator decisions required.
Because AEDs have primarily been designed to treat adult VF and shockable VT, AEDs are typically not recommended for treating pediatric patients. One reason is that pediatric VF is not well documented and understood. For example, the optimal energy required for defibrillating infants and children has not yet been establishedxe2x80x94although currently available information suggests a starting dose of 2 J/kg. Additionally, the criteria used to analyze adult VF would not necessarily be appropriate for pediatric VF because of physiological differences between adults and pediatric patients. Such differences include, for example, heart rate. Finally, the protocol recommended for treating a pediatric victim of sudden cardiac arrest is different than the protocol recommended for treating an adult largely because pediatric VF is typically associated with respiratory failure. (See, Chameides et al. (Eds.) xe2x80x9cPediatric Advanced Life Supportxe2x80x9d (1997-1999) American Heart Assn).
FIG. 1 is a functional block diagram depicting an AED 20 and an electrode unit 21. The electrode unit 21 includes defibrillation electrodes 22 which are connected to a connector 23 by electrode wires 25. In operation, an operator attaches the defibrillation electrodes 22 to a patient 24, and plugs the connector 23 of the electrode unit 21 into a connector 26 of the AED 20. The operator then turns on the AED 20, and ECG signals are gathered by the electrodes 22 and routed to an ECG amplifier 28 within the AED. An A/D converter 30 receives the analog output of the ECG amplifier 28, and provides corresponding digital samples to a microcomputer 32 for analysis. If the patient 24 is currently experiencing VF, the microcomputer 32 asserts a control signal to cause a high voltage charger 34 to transfer electrical energy from a low voltage source, such as a battery 36, to a high voltage energy storage device, such as a capacitor 38. In the case of semi-automatic AEDs, the operator is then prompted by the AED 20 to issue a shock command by depressing a shock control switch 39. In the case of fully automatic AEDs, the shock command is initiated by the microcomputer 32, and no shock control switch 39 is provided. In response to the shock command, the microcomputer operates a discharge switch 40 to deliver an electric shock to the patient 24 through the electrodes 22.
As mentioned above, the use of AEDs for pediatric patients generally has not been considered, primarily because of concerns with potential operator confusion and machine complexity. When defibrillating pediatric patients, the operator must know the appropriate energy dose to deliver, which is based on the pediatric patient""s weight or age. In practical terms, this means that an AED must have the necessary circuitry to accurately produce at least two energy levels (adult and child). Because the AED cannot automatically detect the presence of a pediatric patient, the AED must provide the operator with a means, such as an energy selector switch, to choose the proper energy level. It is also necessary that the AED properly analyze VF in pediatric patient. This may require the AED to be informed, via an operator action, that a pediatric patient is present in order to appropriately modify the ECG analysis to account for the differences between heart rhythms of pediatric and adult patients. The need for an operator to select an appropriate energy level, and to indicate to the AED whether a pediatric or adult patient is present, complicates both the AED design and the operator decision making process each time the AED is used. Added complexity is of particular concern for first responder AEDs which are designed for infrequent use, and are typically used by persons whose primary occupation is not lifesaving (such as police officers or flight attendants). Concerns regarding the possible consequences of such complications have outweighed any expected benefits associated with the small utilization rate of AEDs for pediatric patients. Nevertheless, the inability to effectively treat an infant or child near death is difficult to accept.
What is needed is a simple and effective way of reducing the amount of energy delivered to a pediatric patient by an AED. Additionally, what is needed is a device that lowers the defibrillator energy delivered to a pediatric patient as well as enables the defibrillator to modify its behavior to more effectively treat a pediatric patient. Additionally, what is needed is a device that enables the ECG analysis capabilities to dynamically change when the pediatric energy reduction unit is in place. Finally, what is needed is a simple and effective way of reducing the amount of energy delivered to a pediatric patient by a traditional AED, but which allows a seamless hand-off to a manual defibrillator (or an AED with manual capabilities).
A method and apparatus is provided for reducing energy delivered by an external defibrillator to a child or infant patient. An energy reduction unit is removably connectable to the defibrillator. In one embodiment, the energy reduction unit includes an attenuator which partially dissipates energy produced by the defibrillator. The attenuator may be designed to present an impedance to the AED which, when connected to the patient, is a function of the patient""s impedance. The energy reduction unit may also include a presence-detect function which enables the defibrillator to modify ECG signal analysis to account for differences between heart rhythms of pediatric and adult patients. Additionally, the energy reduction unit may also change the care procedures the defibrillator prompts the rescuer to follow. In a second embodiment, the energy reduction unit includes an energy control modifier circuit which affects the charging operations performed internal to the AED. Once an operator has determined that a pediatric patient falls below a selected measurement threshold level, the operator connects the energy reduction unit to the defibrillator. All other steps performed by the operator are identical to those steps performed when defibrillating an adult.